JPH07110206A - Optical heterodyne interferometer - Google Patents

Abstract

PURPOSE:To provide an optical heterodyne interferometer which is made compact by reducing the number of optical components and is constituted to hardly generate phase distortion by air flows by always bringing two optical paths closer to each other. CONSTITUTION:An acoustooptical modulating element 3 separates laser light by utilizing the Bragg diffraction and, at the same time, modulates high-order modulated light. One of two diffracted light rays of different orders is used as measuring light and the other is used as reference light. Another acoustooptical modulating element 10 modulates the reflected measuring light and reference light respectively reflected from an object 8 to be measured and reference mirror 7 with a frequency which is slightly different from that of the element 3 and generates interference by integrating both light rays. Therefore, the number of optical components used in an optical heterodyne interferometer can be reduced. In addition, since the Bragg angles of the elements 3 and 10 are small, about 7 mrad, the constitution of the interferometer can be made more compact and, since the optical paths for measuring light and reference light can be always brought closer to each other, the phase distortion which is generated by air flows hardly occurs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical heterodyne interferometer, and more particularly to an optical heterodyne interferometer using an acousto-optic modulator.

[0002]

2. Description of the Related Art An example of fine surface shape measurement using an optical heterodyne interferometer will be described.

Conventionally, the optical system of an optical heterodyne interferometer using this type of acousto-optic modulator has been constructed as shown in FIG.

The laser light source 101 generates linearly polarized laser light having a frequency of ν1. Isolator 10
Reference numeral 2 blocks the return light to the laser light source 101, and is set so that the polarization plane of the output light is parallel to the normal line of the paper surface. The non-polarizing beam splitter 103 is
The output light of the isolator 102 is branched in two directions.

The drive signal generator 105 drives the acousto-optic modulator 104 by generating a sine wave of 80 MHz, and the acousto-optic modulator 104 shifts the frequency of one branched laser beam by +80 MHz. It is what makes me. Here, the acousto-optic modulator 104 will be described with reference to FIG. When laser light is incident on the acousto-optic modulator 104 at a Bragg angle φ, 0th-order diffracted light and 1st-order diffracted light having an angle of 2φ with respect to the 0th-order diffracted light are generated. At this time, the frequency of the first-order diffracted light is positively shifted by the frequency generated by the drive signal generator with respect to the incident light. Output light other than the first-order diffracted light is blocked by a light blocking plate or the like, and only the first-order diffracted light enters the acousto-optic modulator 106.

The drive signal generator 107 drives the acousto-optic modulator 106 by generating a 79.9 MHz sine wave, and the acousto-optic modulator 106 directs the laser light to the Bragg angle φ as shown in FIG. When incident at, the 0th-order diffracted light and the -1st-order diffracted light having an angle of 2φ with respect to the 0th-order diffracted light are generated. At this time, the frequency of the −1st order diffracted light is minus shifted by the frequency generated by the drive signal generator with respect to the incident light. Output light other than the -1st order diffracted light is blocked by a light shield plate,
Only the first-order diffracted light is output. Therefore, the laser light transmitted through the two acousto-optic modulators 104 and 106 has a frequency ν
2 is shifted by + 100kHz. Hereinafter, this output laser light will be referred to as reference light. The reflecting mirror 108 changes the optical path of laser light.

The reflecting mirror 109 changes the optical path of the other laser beam split by the non-polarizing beam splitter 103. The half-wave plate 110 has an optical axis set at an angle of 45 degrees with respect to the paper surface of FIG. 4, and rotates the polarization plane of the laser light so as to be parallel to the paper surface. Hereinafter, this output laser light will be referred to as measurement light.

The non-polarizing beam splitter 111 integrates the reference light and the measurement light on the same optical path, and divides them into two beams.
It divides two laser beams. After that, the analyzer 113
The reference light branched in the direction is called the S1 wave, the measurement light is called the P1 wave, the reference light branched in the direction of the polarization beam splitter 115 is called the S2 wave, and the measurement light is called the P2 wave.

The analyzer 113 causes the S1 wave and the P1 wave to interfere with each other. The photodiode 114 is
The intensity of this interference light is converted into an electric signal. This electric signal is a reference signal for detecting a phase difference, which will be described later, and is hereinafter referred to as a B signal.

The polarization beam splitter 115 has a property of reflecting the S-polarized wave and straightening the P-polarized wave. Therefore, the S2 wave is reflected in the direction of the quarter-wave plate 116, and P2
The wave transmits in the direction of the quarter wave plate 118. The S2 wave is reflected by the fixed reference mirror 117 and returns to the polarization beam splitter 115, but by passing through the quarter wavelength plate 116 twice, it becomes a P polarization wave and is transmitted in the direction of the analyzer 120. The P2 wave is reflected by the DUT 119 and returns to the polarization beam splitter 115.
By passing 8 times twice, it becomes an S-polarized wave and the analyzer 1
Change direction to 20.

The analyzer 120 causes the S2 wave and the P2 wave to interfere with each other, and the photo diode 121
Is for converting the intensity of the interference light into an electric signal. Here, S input to the photodiode 121
2 waves, Aexp {i (k2 ・ rs-ω2 ・ t)} (where rs is the optical path length of the S2 wave to the photodiode 121, A is the amplitude, k
2 is a wavelength constant, ω2 = 2π · ν2 is an angular frequency, and P2 wave is B · exp {i (k1 · rp-ω1 · t)} (however, rp is a P2 wave up to the photodiode 121). The optical path length, B is the amplitude, k1 is the wavelength constant, ω1 = 2π · ν1 is the angular frequency, and the initial phase is 0).
| ² = A ² + B ² + 2A ・ B ・ cos {(ω2-ω1) ・ t + k1 ・ rp-k2 ・ rs} is converted into an electric signal. The third term is a beat signal due to the interference of the S2 wave and the P2 wave, and its phase θ is k1 · rp−k2 · rs. By fixing the reference mirror 117, the minute displacement of the DUT 119 can be measured from the phase θ of the beat signal. This electrical signal is hereinafter referred to as the D signal.

Next, the structure of the electric system will be described with reference to FIG.

The D signal amplifier 131 amplifies the AC component (beat signal) of the D signal in the previous period.

Similarly, the B signal amplifier 132 amplifies the beat signal of the B signal in the previous period. At this time, the phase of the beat signal is the photodiode 114 of the S1 wave and the P1 wave.
Since the optical path lengths up to are fixed, they are constants and are used as a comparison reference for the phase of the D signal described below.

The phase comparator 133 detects the phase difference Θ between the beat signals of the D signal and the B signal. The phase difference Θ becomes Θ = k1rp-k2rs- (phase of the B signal) as described above, and by fixing the reference mirror 117, Θ = k1rp-
(Constant).

The signal processing device 134 gradually moves the stage on which the DUT 119 is placed in the direction of the arrow in FIG.
By observing the change in the phase difference Θ, the measured object 11
The fine surface shape of No. 9 is measured. At this time, the measurement accuracy is Δrp = ΔΘ / k1 = λ / 2π · ΔΘ (where λ is the laser wavelength). Here, if a phase difference Θ is detected with an accuracy of 2π / 1000 using a He-Ne laser with a wavelength of 632.8 [nm],
Δrd = 0.6 [nm], and the optical path of the measurement light is 119
Since it reciprocates a distance corresponding to the displacement of
The displacement of can be measured with an accuracy of 0.3 [nm].

[0017]

However, in the optical heterodyne interferometer using the acousto-optic modulator as described above, in order to produce two laser beams having slightly different wavelengths, the linearly polarized laser beam is a non-polarized beam. Splitter 103
And the frequency of one of the laser beams is shifted by two acousto-optic elements, and then integrated into the same optical path by the non-polarization beam splitter 111, the number of optical components increases and the non-polarization Beam splitter 103
Therefore, there is a problem that the optical paths of the reference light and the measurement light are separated between the non-polarization beam splitters 111, and the phase distortion due to the air flow is likely to occur.

The present invention has been made in order to solve the above-mentioned problems. The number of optical components is reduced, the size is made compact, and the optical paths of the reference light and the measurement light are always brought close to each other, so that the phase distortion due to the air flow is reduced. The object is to provide an optical heterodyne interferometer that is hard to generate.

[0019]

In order to achieve this object, an optical heterodyne interferometer according to the present invention splits a laser beam by Bragg diffraction and modulates a high-order diffracted light to obtain two diffracted lights of different orders. A first acousto-optic modulator using one of them as a measurement light and the other as a reference light; a reference light reflection means for reflecting the reference light; an object to be measured; and the reflection light reflected by the reference light reflection means. A second acousto-optic modulator that modulates the measurement light and the reference light at a frequency slightly different from that of the first acousto-optic modulator, and integrates and interferes by Bragg diffraction is provided.

In the optical heterodyne interferometer according to the second aspect, the measurement light is the 0th-order diffracted light by the first acousto-optic modulator, and the reference light is the 1st-order diffracted light.

[0021]

In the optical heterodyne interferometer of the present invention having the above-mentioned structure, the first acousto-optic modulator splits the laser beam by Bragg diffraction and modulates the high-order diffracted light to obtain two diffracted lights of different orders. One of them is the measuring light,
The other is used as the reference light.

The reference light reflecting means reflects the reference light.

The second acousto-optic modulator modulates the DUT, the measurement light reflected by the reference-light reflecting means, and the reference light at a frequency slightly different from that of the first acousto-optic modulator. At the same time, they are integrated and interfered by Bragg diffraction.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to the drawings. Further, in the examples, description will be given by taking as an example the measurement of a fine surface shape using the optical heterodyne interferometer of the present invention.

FIG. 1 schematically shows the structure of an optical system of an optical heterodyne interferometer embodying the present invention.

The laser light source 1 generates a linearly polarized laser light having a frequency of ν1. The isolator 2 blocks the return light to the laser light source 1, and is set so that the polarization plane of the output light is parallel to the normal line of the paper surface.

The drive signal generator 4 drives the acousto-optic modulator 3 by generating a sine wave of 80 MHz from the reference clock of the crystal oscillator 5.

The acousto-optic modulator 3 splits the linearly polarized laser light by Bragg diffraction and shifts the frequency of the first-order diffracted light by +80 MHz. Here, the acousto-optic modulator 3 will be described with reference to FIG. When the laser light is incident on the acousto-optic modulator 3 at the Bragg angle φ, the 0th-order diffracted light and the 1st-order diffracted light having an angle of 2φ with respect to the 0th-order diffracted light are generated as shown in FIG.
At this time, the frequency of the first-order diffracted light is positively shifted by the frequency generated by the drive signal generator with respect to the incident light.
The 0th-order diffracted light is output at the same frequency as the incident light. Output light other than the 0th-order diffracted light and the 1st-order diffracted light is blocked by a light blocking plate or the like.
Hereinafter, the output 0th-order diffracted light is referred to as measurement light and the 1st-order diffracted light is referred to as reference light.

Since the measuring light and the reference light are S-polarized waves, they are reflected by the polarization beam splitter 6.

The measurement light and the reference light are reflected by the DUT 8 and the reference mirror 7 and return to the polarization beam splitter 6. At this time, the measurement light and the reference light are transmitted through the quarter-wave plate 9 twice, so that P It becomes a polarized wave and goes straight through the polarization beam splitter 6.

The drive signal generator 11 drives the acousto-optic modulator 10 by generating a 79.9 MHz sine wave from the reference clock of the crystal oscillator 5.

When the measurement light and the reference light are incident at the Bragg angle φ as shown in FIG. 2, the acousto-optic modulator 10 causes the 0th-order light of the measurement light and the -1st-order diffracted light of the reference light to be on the same optical path. It integrates and interferes. At this time, the frequency of the reference light is
-79.9MHz shift. Therefore, by passing through the two acousto-optic modulators 3 and 10, the frequency ν of the reference light is
2 is shifted by + 100kHz.

The photo diode 12 converts the intensity of the interference light into an electric signal. Here, the reference light input to the photodiode 12 is changed to A · exp {i (k2 ·
rr-ω2 · t)} (where rr is the optical path length of the reference light to the photodiode 12, A is the amplitude, k2 is the wavelength constant, and ω2 = 2π
・ Ν2 is the angular frequency) and the measurement light is B ・ exp {i (k1 ・ rd-ω1
・ T)} (where rd is the photodiode 12 of the measuring light)
Optical path length to, B is the amplitude, k1 is a wavelength constant, .omega.1 = 2 [pi-.nu.1 is the angular frequency, the initial phase is 0) to the photo-diode 12, the intensity of the incident light ^{| I | 2 = A 2 +} B ² + 2A ・ B ・ cos {(ω
2-ω1) · t + k1 · rd-k2 · rr} is converted into an electric signal. Third
The term is a beat signal due to the interference between the measurement light and the reference light, and its phase θ is k1 · rd−k2 · rr. By fixing the reference mirror 7, it is possible to measure the minute displacement of the DUT 8 from the phase θ of the beat signal. This electrical signal will be referred to as D
Call it a signal.

Next, the configuration of the electric system will be described with reference to FIG.

The D signal amplifier 31 amplifies the AC component (beat signal) of the D signal in the previous period.

The B signal generator 32 is a reference signal of 100 KHz from the reference clock of the crystal oscillator 5 in the previous period (hereinafter, B signal).
Is generated. This signal is used as a comparison reference for the phase of the D signal.

The phase comparator 33 detects the phase difference Θ between the beat signal of the D signal and the B signal. Phase difference Θ
Becomes Θ = k1rd-k2rr- (phase of the B signal) as described above. By fixing the reference mirror 7, Θ = k1rp-
(Constant).

The signal processing device 34 gradually moves the stage on which the DUT 8 is placed in the direction of the arrow in FIG. 1 and observes the change in the phase difference Θ to determine the fine surface shape of the DUT 8. It is something to measure. At this time, the measurement accuracy is Δr
d = ΔΘ / k1 = λ / 2π · ΔΘ (where λ is the laser wavelength). If a phase difference Θ is detected with an accuracy of 2π / 1000 using a He-Ne laser having a wavelength of 632.8 [nm], then Δrd = 0.
Since it becomes 6 [nm] and the optical path of the measurement light reciprocates the distance corresponding to the displacement of the DUT 8, the displacement of the DUT 8 is 0.3 [nm].
It can be measured with the accuracy of.

As described above in detail, the frequency of the laser light is shifted by the two acousto-optic modulators, and the measuring light and the reference light are branched and integrated, whereby the number of optical components of the optical heterodyne interferometer is increased. Can be reduced. Moreover, since the Bragg angle of the acousto-optic modulator is as small as about 7 [mrad], the optical heterodyne interferometer can be made compact, and the optical paths of the measurement light and the reference light can always be close to each other. Less likely to occur.

In the above embodiment, the measurement of the fine surface shape is explained as an example, but other measurement using an optical heterodyne interferometer may be used.

[0041]

As is apparent from the above description, according to the optical heterodyne interferometer of the present invention, the first acousto-optic modulator splits the laser light by Bragg diffraction and modulates the high-order diffracted light. Then, the 0th order diffracted light is measured light, 1
The second diffracted light is used as the reference light, and the measurement light and the reference light reflected by the DUT and the reference light reflecting unit have a frequency at which the second acousto-optic modulator slightly differs from the first acousto-optic modulator. It is possible to reduce the number of optical components by modulating the light with the optical system and integrating and interfering by Bragg diffraction. The Bragg angle of the acousto-optic modulator is about 7 [mra
Since d] is small, the optical heterodyne interferometer can be made compact, and the optical paths of the measurement light and the reference light can always be close to each other, and phase distortion due to the air flow is unlikely to occur.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an optical system of the present embodiment.

FIG. 2 is an explanatory diagram of an acousto-optic modulator according to an embodiment.

FIG. 3 is a block diagram showing an electrical control configuration of the present embodiment.

FIG. 4 is a schematic configuration diagram of an optical system of a conventional optical heterodyne interferometer.

FIG. 5 is an explanatory diagram of an acousto-optic modulator.

FIG. 6 is an explanatory diagram of an acousto-optic modulator.

FIG. 7 is a block diagram showing a conventional electrical control configuration.

[Explanation of symbols]

 3 Acousto-optic modulator 7 Reference mirror 10 Acousto-optic modulator

Claims (2)

[Claims] 1. A first acousto-optic modulator that splits laser light by Bragg diffraction and modulates higher-order diffracted light, and uses one of two diffracted lights of different orders as measurement light and the other as reference light. A reference light reflecting means for reflecting the reference light, an object to be measured, the measurement light reflected by the reference light reflecting means, and the reference light having a frequency slightly different from that of the first acousto-optic modulator. An optical heterodyne interferometer, comprising: a second acousto-optic modulator which is modulated by the optical system and integrated by Bragg diffraction to cause interference. 2. The optical heterodyne interferometer according to claim 1, wherein the measurement light is 0th order diffracted light by the first acousto-optic modulator and the reference light is 1st order diffracted light.